In present digital cellular communication systems a speech encoder rate is chosen at call setup and is fixed throughout the call. This strategy is used in the D-AMPS and GSM systems. However, this strategy gives good performance with respect to channel robustness and speech quality over only a limited range of channel conditions.
In CDMA systems, such as IS-95, there is a variable speech and channel encoder scheme, but the speech and channel coder mode decision for one link is based on speech signal characteristics and is independent of the mode of a return link.
It has also been suggested as described in Dunlop J, Irvine J and Cosimini P. "Estimation of the Performance of Link Adaptation in Mobile Radio", IEEE Vehicular Technology Conference, VTC-95, Chicago 1994, to use dynamically changing encoding modes with different mixes of speech/channel encoding suitable for different radio environments. However, these methods are too slow, since they need to keep signaling overhead as low as possible.
WO 94/07313 assigned to ANT NACHRICHTENTECHNIK GMBH, and Yuen E, Ho P and Cuperman V, "Variable Rate Speech and Channel Coding for Mobile Communication", IEEE Vehicular Technology Conference, VTC-95, Stockholm 1994 describe systems in which a receiver may request a mode change from a transmitter over a dedicated back channel.
It would be desirable to dynamically allocate speech and channel coder rates depending upon local radio conditions on a frame by frame basis in order to achieve a close to optimum performance in a larger range of conditions, and to let the coding mode of one link control the allocation of coding mode for the return link.